1. Field of the Invention
The present invention relates to a heat shrinkable polyester film, and particularly to a heat shrinkable polyester film suitable for use in a label. More particularly, the present invention relates to a heat shrinkable polyester film for use in a label with high compressive strength, which has very few creases, uniform shrinkage, and minimal distortion occurring from a heat shrinkage process.
2. Description of the Related Art
Heat shrinkable polyester films made of polyvinyl chloride, polystyrene, or the like, are widely used in labels wrapped around the body of polyethylene terephthalate (PET) bottles. However, polyvinyl chloride products pose a hazard as they generate a chlorine gas when incinerated, and polystyrene products cause problems such as printing difficulties. Moreover, in order to recycle PET bottles, it is necessary to separate non-PET labels thereon which are made of polyvinyl chloride, polystyrene, etc., from the PET bottles. Thus, heat shrinkable films made of polyester, which are free of the above-mentioned problems, have been attracting public attention.
However, heat shrinkable polyester films have not been effective as shrinkable films for use in labels, because many of them shrink too rapidly resulting in creases, non-uniform shrinkage and distortion after the shrinkage. Moreover, such films are easily torn by an externally applied force after the shrinkage.
In order to avoid some of these problems, Japanese Patent Publication for Opposition No. 7-77757 discloses a method for improving a final shrinkage property by remarkably reducing a tearage strength along a direction perpendicular to its main shrinkage direction.
Moreover, Japanese Laid-Open Publication No. 58-64958 discloses a method for improving a final shrinkage property by reducing an orientation returning stress.
However, the films obtained by the above-mentioned methods could not achieve sufficient final shrinkage properties for use with small PET bottles whose shrink tunnel passing time is short. Thus the films were ineffective as shrinkage films. That is, when placing a tubular shape label, which is formed by a shrinkable film, around a PET bottle and performing a heat treatment to shrink the film onto the PET bottle, crease, non-uniform shrinkage, and distortion have sometimes occurred from the film shrinkage process.
In addition, the process speed for beverage bottling lines for PET bottles, and the like, is increasing. Therefore, the label is required to have a high-speed application property as well as a good final shrinkage property as mentioned above. In short, as illustrated in FIGS. 1 and 2, when a label 2 is placed around a PET bottle 1 in a high-speed process by a pressure member 3, if the high-speed application property is poor, the label cannot be applied properly due to insufficient rigidity of the label. The application property of the label is likely to depend greatly on the rigidity of the film, whereby it is possible to address such a problem by increasing a film thickness. However, problems occur when the film thickness increases. For example, a film thickness increase leads to an increase in weight, which results in a poor handling property. There are also other problems such as the film thickness increase leading to higher costs.
According to one aspect of this invention, there is provided a heat shrinkable polyester film whose hot water shrinkage along its main shrinkage direction is equal to or greater than 20% when the film is put in hot water of 70xc2x0 C. for 5 sec; 35% to 55% when put in hot water of 75xc2x0 C. for 5 sec; and 50% to 60% when put in hot water of 80xc2x0 C. for 5 sec, wherein the compressive strength of the label formed by the film satisfies a following expression (1):
y greater than x2.2xe2x80x83xe2x80x83(1)
where y denotes compressive strength (mN) and x denotes film thickness (xcexcm).
In one embodiment of the invention, the film has a thickness distribution of 6% or less.
According to another aspect of this invention, there is provided a label for a bottle which is produced by a heat shrinkable polyester film of the present invention.
Thus, the invention described herein makes possible the advantages of providing a heat shrinkable polyester film for use in various applications including a small PET bottle, with a good high-speed application property and a good final shrinkage property, without exceeding the film thickness of a conventional film.